<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<meta charset="UTF-8">
<title>计算圆的周长和面积</title>
<body>
    <div>
        <p>圆的半径：<input id="r" type="text"></p>
        <p>圆的周长：<input  id="cir" type="text"></p>
        <p>圆的面积：<input  id="area" type="text"></p>
    </div>
<script>
var r=prompt('请输入圆的半径');
r=parseFloat(r)&&Number(r);
if(!isNaN(r)){
    var cir=2*Math.PI*r;
    var area=Math.PI*r*r;
    document.getElementById('r').value=r;
    document.getElementById('cir').value=cir.toFixed(2);
    document.getElementById('area').value=area.toFixed(2);
}else{
    alert('请输入正确的数字');
}
</script>
</body>
</html>